A variety of methods have heretofore been used or proposed for use in applying metallic platings to all or portions of the surfaces of polymeric plastic parts. Such processes conventionally comprise a plurality of sequential pre-treatment steps to render the plastic substrate receptive to the application of electroless plating whereafter the plated part can be processed through conventional electroplating operations to apply one or a plurality of supplemental metallic platings over all or selected portions of the plastic substrate. Conventionally, the pre-treatment steps employed include a cleaning or series of cleaning steps, if necessary, to remove surface films or contaminating substances, followed thereafter by an aqueous acidic etching step employing a hexavalent chromium solution to render the plastic hydrophillic and create bonding sites to achieve a desired surface roughness or texture enhancing a mechanical interlock between the substrate and the metallic plating to be applied thereover. The etched substrate is then subjected to one or a plurality of rinse treatments to extract and remove any residual hexavalent chromium ions on the surfaces of the substrate which may also include a neutralization step including reducing agents to substantially convert any residual hexavalent chromium ions to the trivalent steps. The rinsed etched substrate is thereafter typically subjected to an activation treatment in an aqueous acidic solution containing a tin-palladium complex to form active sites on the surface of the substrate followed by one or more rinsing steps after which the activated surface is typically subjected to an accelerating treatment in an aqueous solution to extract any residual tin constituents or compounds on the surface of the substrate and thereby expose active catalytic sites. The accelerated plastic part is again water rinsed and thereafter is subjected to an electroless plating operation of any of the types known in the art to apply a metallic plate such as copper, nickel, or cobalt over all or certain selected areas thereof whereafter the part is rinsed and thereafter is subjected to conventional electroplating operations.
Typical of such plastic plating processes are those described in U.S. Pat. Nos. 3,011,920; 3,257,215; 3,259,559; 3,310,430; 3,329,512; 3,377,174; 3,532,518; 3,615,736; 3,622,370; 3,961,109; 3,962,497; 4,153,746; and 4,204,013; as well as those described in articles entitled "Stabilizing Electroless Copper Solutions", by E. B. Saubestre, Plating, June, 1972; and "Improvements in Electroless Copper for Automotive Plastic Trim", by D. A. Arcilesi, Plating and Surface Finishing, June, 1981; as well as those described in my copending application entitled "Metallic Impurity Control for Electroless Copper Plating", Ser. No. 314,280 filed Oct. 23, 1981; to which reference is made for further details of the processes, and the disclosures of which are hereby incorporated by reference. The present invention is believed to be applicable to processes of the foregoing type and is specifically directed to an improved electroless copper plating rate controller which provides benefits and advantages heretofore unattainable in accordance with prior art practices.
In a conventional electroless copper plating bath, the various components of the plating bath are aqueous concentrates, and include such basic components as copper concentrate, a metal solubilizer or complexer, a reducing agent, and a pH adjuster. In addition, a stabilizer and a plating rate controller may also be used. Most of the early electroless copper processes used cupric sulfate as the source of metal ions. However, more recent processes employ cupric chloride, which is more soluble than copper sulfate. Due to the high alkalinity of present state-of-the-art autocatalytic copper baths, a complexer is needed to prevent the precipitation of copper as its hydroxide. Substituted aliphatic amine chelating agents such as ethylenediaminetetraacetic acid tetrasodium salt (Na.sub.4 EDTA) have been found to be effective copper solubilizers over relatively broad pH and temperature ranges, and therefore are widely used. Formaldehyde (such as a 37 percent solution stabilized with 10 percent methanol) is believed to be the major reducing agent used in high volume production installations. Sodium hydroxide solutions (50 percent caustic soda for example) are used to maintain the pH at from about 11 to 13, depending on the specific additive system being used. It is important to control the pH carefully because the ability of formaldehyde to reduce copper increases dramatically with increasing pH.
Because copper is autocatalytic, random copper particles that form in solution would be plated indefinitely if they were not stabilized. An electroless copper stabilizer causes the plating rate at a given copper surface to diminish as plating time increases. Among the reasons for using a stabilizer are to limit metal deposition to the work being plated and to prevent solution decomposition. If no stabilizer were present, copper particles or solid impurities falling to the bottom of the plating tank would be plated. Furthermore, they would continue to plate in an uncontrolled manner until the solution decomposed due to massive tank plating. Some stabilizers can also improve the luster and/or ductility of copper deposits.
Electroless copper stabilizers are compounds that cause the formation of non-catalytic thin films on the surface of electroless copper deposits that remain in the solution for extended periods of time. Heterocyclic organic sulfur compounds are believed to be the most widely used electroless copper stabilizers. They have replaced numerous other organic and inorganic sulfur compounds, including colloidal sulfur. Very high molecular weight organic polymers such as gelatin, hydroxy alkyl starches, cellulose ethers, polyamides, polyvinyl alcohol, and polyalkylene oxides have also been used to encapsulate copper particles.
Rate controllers such as cyanide iodide, or other related organic compounds, and nonsulfur containing nitrogen heterocyclics such as bipyridyls and phenanthrolines, reduce the activity of electroless copper processes. Rate controllers are used to reduce the rate of the electroless copper reduction reaction, thereby regulating the copper plating thickness per unit time. Rate controllers also accommodate stabilizers and help them function better by giving them more time to form noncatalytic coatings over the active plating sites in view of the decreased plating rate. It is known that the reduction of cupric ions to copper metal is a two-step process in which the divalent copper is first reduced to monovalent copper (the rate determining step in the absence of rate controllers and stabilizers), and then to copper metal. In view of this two-step reduction, and given that rate controllers are generally inorganic or organic substances which form more stable complexes with monovalent copper than with divalent copper, it follows that rate controllers lower the plating rate by retarding the conversion of monovalent copper to copper metal. With regard to conventional rate controllers such as cyanide, for example, very small amounts of cyanide ions can reduce the plating rate significantly, but substantial increases in the amount of cyanide ions over such small amounts will generally not cause any significant additional rate change. Therefore, although cyanide compounds are generally effective over a wide concentration range and are relatively easy to control, they provide non-linear control, which is often undesirable because intermediate plating rates between the high and low values cannot be effectively achieved by varying the concentration of the cyanide compounds.
If no rate controller was used, it would be virtually impossible, at least in most commercial applications, to achieve adequate filtration of the plating solutions to remove particles that would form at a high rate and subsequently cause the decomposition of the plating solution due to massive copper nucleation throughout the solution. In addition to providing a controlled electroless copper reaction rate, rate controllers can also improve the luster and ductility of copper deposits by acting as grain refiners to produce smoother, brighter, less porous, denser deposits.
At the present time, it is believed that the most widely used rate controllers are cyanide or organic derivatives of cyanide, all of which are toxic. Thus a continuing problem associated with the use of such cyanide-type rate controllers has been the control and/or care necessary in the handling and use of such materials and resulting electroless copper plating solutions. Likewise, such cyanide-type rate controllers and resulting electroless copper plating solutions require special consideration as far as environmental factors are concerned, especially with regard to waste treatment and disposal. Accordingly, a need existed for a non-toxic, environmentally acceptable rate controller for use in electroless copper plating solutions and processes, which rate controller would also be stable, easy to control, and adapted for use with current conventional electroless copper plating systems.